Field of the Invention
The present invention relates to an image reading apparatus capable of preventing temperature increase of a light source for irradiating a document with light and of a reading sensor.
Description of the Background Art
Generally, in an image reading apparatus, a document surface is irradiated with light emitted from a light source, the light reflected from the document surface is directed to an image pick-up surface of a reading sensor using an optical system including a lens or the like to form an image, and thereby, an image on the document is read.
In such an image reading apparatus, the light source is kept on during an image reading period. Therefore, during the image reading period, the light source generates heat and its temperature increases. Typically, the light source and the reading sensor are placed in an enclosure for preventing entrance of dust and, therefore, temperature in the enclosure of image reading apparatus tends to increase because of heat generated by the light source and, naturally, the temperature of reading sensor also increases.
As the reading sensor, a CCD (Charge Coupled Device) image sensor is generally used. In the image reading period, when a sensor driving circuit driving the CCD image sensor enters an operating state, clocks are supplied from the sensor driving circuit to the CCD image sensor. Therefore, in the image reading period, the temperature is higher than in a standby state, because of self-heating of the reading sensor.
When the temperature of reading sensor increases, sensitivity of the reading sensor may vary, or members forming the reading sensor may deform because of thermal expansion, possibly causing problems such as deviation of a point of focus of the image forming system, or deviation of reading position. Therefore, conventionally, a cooling mechanism has been provided in the enclosure of image reading apparatus, and the light source lamp for the document and the reading sensor are cooled only during the reading period. By way of example, Japanese Patent Laying-Open No. 8-102822 (hereinafter referred to as '822 Reference) discloses a technique of arranging a cooling fan on a wall surface of an enclosure, and by introducing external air from the outside into the enclosure, cooling the light source lamp for the document and the reading sensor. By such an approach, during the image reading period, temperatures of various members including the reading sensor are maintained close to a prescribed temperature.
As described above, a reading unit for reading a document is in most cases formed to have a sealed structure to prevent entrance of paper powder or dust, and to prevent any foreign material from adhering to optical components and the like arranged therein. With the reading unit having such a sealed structure, the temperature of light source and its vicinity in the reading unit attains very high. If an opening is provided at the reading unit to prevent temperature increase, however, there arises a problem that inside of the scanner (image reading apparatus) is contaminated by dust or foreign matters. The problem of scanner contamination is similarly experienced if the external air is introduced to the inside of enclosure to cool the light source lamp for the document and the reading sensor, as in '822 Reference.
It may be possible to provide a fan on a wall surface of a scanner frame, to discharge air that is heated to a high temperature in the scanner. The scanner unit, however, is substantially sealed and, hence, it is difficult to efficiently discharge high-temperature air in the scanner. Therefore, sufficient cooling effect cannot be attained. If an external air inlet is provided, the efficiency of discharge can be increased, whereas dust-proof property degrades.